A metal-insulator-metal capacitor is widely used. For different applications, the requirement to the breakdown voltage of the metal-insulator-metal capacitor is entirely different. For example, please refer to FIGS. 6, 6A and 6B (that are respectively the FIG. 5, FIG. 11 and FIG. 4 of “Berthelot, A.; Caillat, C.; Huard, V.; Barnola, S.; Boeck, B.; Del-Puppo, H.; Emonet, N. & Lalanne, F. (2006). Highly Reliable TiN/ZrO2/TiN 3D Stacked Capacitors for 45 nm Embedded DRAM Technologies, Proc. of ESSDERC 2006, pp. 343-346, Montreux, Switzerland, September 2006”), which shows that the requirement to the breakdown voltage of the conventional metal-insulator-metal capacitor for the application to the dynamic random access memory (DRAM) is usually lower than 6V.
Please refer to FIG. 6, which is the design of the metal-insulator-metal capacitor for the application of DRAM. The design comprises a TiN bottom metal layer, a ZrO2 dielectric layer and a TiN top metal layer. The dielectric layer may be made of HfO2 or Al2O3. Please also refer to FIGS. 6A and 6B. As an example the dielectric layer is made of Al2O3, when the thickness of the Al2O3 film is approaching 50 Å (the effective oxide (SiO2) thickness is about 20 Å), the breakdown voltage of the capacitor is about 5.5V. While in the case of HfO2 dielectric layer and ZrO2 dielectric layer, the breakdown voltage is even lower than 4V. However, on the application of mobile phone circuit, the requirement to the breakdown voltage of the metal-insulator-metal capacitor is up to 50V, which is far beyond the requirement on the application of DRAM.
Not only the gap difference on the requirement of the breakdown voltage, but also the requirement of the lifetime on the application of DRAM is quite different from the requirement on the application of the mobile phone. Time dependent dielectric breakdown lifetime test is the method often used in semiconductor industry to test the reliability of the integrated circuits devices. During testing the metal-insulator-metal capacitor, under high temperature environment and under applying a certain fixed stress voltage on the capacitor, measures the time interval when breakdown occurs. Estimate the lifetime of the capacitor by extrapolation to judge if it fits the requirement of the lifetime test. For DRAM application metal-insulator-metal capacitor, the required lifetime is usually greater than or equal to 10 years (3.1*108 sec) under applying 3V stress voltage; while for mobile phone application metal-insulator-metal capacitor, the required lifetime should be greater than or equal to 20 years (6.3*108 sec) under applying 20V stress voltage and at temperature 125° C. Obviously, the design of the metal-insulator-metal capacitor for DRAM application couldn't meet the requirement for mobile phone application no matter the requirement of lifetime or the breakdown voltage.
Please refer to FIG. 7, which is the sectional view of a prior art metal-insulator-metal capacitor for mobile phone application. On a GaAs substrate, in sequence an isolation layer, a first metal layer (Au), a Si3N4 dielectric layer, an adhesion layer (Ti) and a second metal layer (Au) are formed. When the thickness of the Si3N4 dielectric layer is 1000 Å, after the time dependent dielectric breakdown lifetime test under applying 20V stress voltage, the lifetime may be greater than or equal to 20 years (6.3*108 sec) and may reach the requirement. Its breakdown voltage is 81.5V, which also reaches the requirement for mobile phone application. However, its capacitance density is only 580 (pF/mm2) For dense integrated circuit design, it needs a new design of the metal-insulator-metal capacitor with higher capacitance density.
The design with higher capacitance density means the capacitor having a higher capacitance per unit area. In other words, when the circuit needs a certain fixed capacitance of capacitor, the higher capacitance density the metal-insulator-metal capacitor design has, the smaller area the capacitor needs. For example, when designing the mobile phone circuit, using metal-insulator-metal capacitor design with higher capacitance density will significantly shrink the area of the capacitor. Because a large portion of the area on the mobile phone circuit is occupied by the capacitor, a design of the metal-insulator-metal capacitor with high capacitance density will be able to largely reduce the costs and make the products more competitive.
Accordingly, the inventor develops a new design to overcome the drawback of the conventional metal-insulator-metal capacitor design. The new design of the metal-insulator-metal capacitor passes the time dependent dielectric breakdown lifetime test and has a high capacitance density. Under applying 20V the stress voltage, its lifetime is greater than or equal to 20 years (6.3*108 sec) and reach the requirement. In the meanwhile, its breakdown voltage is higher than 50V and reaches the requirement for mobile phone application.